1. Field of the Invention
This invention relates to ethylene-vinyl alcohol (EVA) copolymer membranes useful as separation membranes having good water permeability and more particularly to novel EVA copolymer membranes comprising a skin layer and a porous layer having a specific structure.
2. Description of the Prior Art
Various membranes have heretofore been developed as dialysis or ultrafiltration membranes for medical and industrial uses. The present inventors have been searching for EVA copolymer membranes having good biocompatibility, good antihemolytic and antithrombotic properties, good durability and good chemical stability, and have already developed an EVA membrane having a homogeneous structure.
The previously developed homogeneous structure EVA membrane has an excellent performance for a dialysis membrane and is being put to practical use as a membrane for artificial kidneys. It has a structure formed by particles having an average diameter in the range of 100-10,000 Angstrom units bonded to each other, said structure being substantially free from pores in excess of 2 microns in diameter.
Generally, in addition to the homogeneous structure membrane such as mentioned above, there is another kind of membrane, which is the so-called asymmetrical membrane having a skin layer. While asymmetrical membranes made from cellulose acetate or polyacrylonitrile are known in the art, asymmetrical membranes from EVA polymers are scarecely known. One example of an asymmetrical EVA membrane appears in Japanese Patent Application Laid Open No. 53-77833, which discloses asymmetrical type EVA membranes for use as separator membranes for storage batteries. This membrane comprises a skin layer and a porous supporting layer thereunder. However, said porous layer, i.e. the supporting layer, has a honeycomb structure with pore sizes of 0.05-10 microns, said structure being essentially different from the structure of the membrane provided by the present invention.
The present inventors have endeavored to obtain an EVA copolymer membrane with a more improved water permeability, and have now succeeded in obtaining an asymmetrical membrane having large vacuoles or voids in the porous layer, and a high porosity.